Welcome to the Lightroom Community. I'll help you figure this out. I'll need more info.
Could you please share the version of Lightroom & the iOS your iPhone is working on? Does this happen with a specific video, or do all videos behave similarly?
Please log on to Lightroom Web (https://lightroom.adobe.com) & try to export videos using Lightroom Web as a workaround and let us know if it works.